xtensa: Provide dummy dma_alloc_attrs() and dma_free_attrs()

xtensa:allmodconfig fails to build with the following errors.

drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:
	In function ‘gk20a_instobj_dtor_dma’:
drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:154:2: error:
	implicit declaration of function ‘dma_free_attrs’
drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:
	In function ‘gk20a_instobj_ctor_dma’:
drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:218:2: error:
	implicit declaration of function ‘dma_alloc_attrs’

Xtensa does not provide those functions at this time.
Provide dummy implementations to avoid build errors.
Acked-by: NMax Filippov <jcmvbkbc@gmail.com>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NChris Zankel <chris@zankel.net>

arch/xtensa/include/asm/dma-mapping.h

@@ -185,4 +185,17 @@ static inline int dma_get_sgtable(struct device *dev, struct sg_table *sgt,
 	return -EINVAL;
 }
 
+static inline void *dma_alloc_attrs(struct device *dev, size_t size,
+				    dma_addr_t *dma_handle, gfp_t flag,
+				    struct dma_attrs *attrs)
+{
+	return NULL;
+}
+
+static inline void dma_free_attrs(struct device *dev, size_t size,
+				  void *vaddr, dma_addr_t dma_handle,
+				  struct dma_attrs *attrs)
+{
+}
+
 #endif	/* _XTENSA_DMA_MAPPING_H */